Sonicwall not passing WAN ip on outbound traffic

I have a Sonicwall tz-170 The wan ip adress is XXX.XXX.XXX.73 . This is the adress that my MX record for inbound mail translated to via a NAT . Inbound mail works fine. Upon sending an email message, I notice the "sending" address is XXX.XXX.XXX.78 which is my DSL Motorola router. The router is using VIP to pass through all traffic to the SOnicwall. If a mail server is using RDNS , the record comes back to the .73 address and fails. Is there a way to force the outbound traffic to show as coming from the Wan ip of the Sonicwall rather than the Wan ip of the Motorola router ???